DECEMBER 2012

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THE AIR COOLER “CONUNDRUM”

Here is an interesting engineering problem, which many of us encountered, and certainly wondered, what the right answer is.

Evoporative air coolerA cheap, water cooled cooler is used in a room, supposedly helping to cool the room.

The sellers sweet talked the buyers into buying, saying it will cool their room, cheaply. Do their claims have any merit?

This is an engineering problem, and we have done the calculations and the answer is surprising.

Picture 1

The user adds water to the water tank, and run the fan. The fan would draw air in and inside, a piece of rolling cloth allows the air to pass through it, supposedly cools the air down. When water is put inside, and the unit is operated, can anyone actually confirm that they feel the air is colder? Many were not sure, “Kind of feel the same”, many lamented. If the fan is circulating the air faster and stronger, the user would feel colder indeed, but wait, is it the fast moving air kind of feeling, or is it really cool air? Some would even use thermometer in front of the fast moving air, yet they still could not verify whether it is colder as the salespeople said it would. Hardly anyone conduct their test from the beginning, until a couple of hours; many only tested the air just for the first few minutes. Is their observation of some cooling, correct then?

In order to answer this question for good, we have to be a bit technical. But don’t worry, we will keep it simple for you, in simple words.

At the rolling cloth, the flowing air evaporates the water. Evaporating water is much and basically similar to evaporating steam, only at low pressure and temperature.
The evaporation of water at the rolling cloth will cause:

  1.  Hot ‘steam’ of moisture entering the air stream.
  2. Colder water to re-enter the water tank.

The reason is because during evaporation, energy is transferred successfully into the evaporating air, leaving behind colder water droplets. This is also similar to when you are sweating, when your sweat evaporates, it will make you feel colder, and it does not matter whether the air is blowing or not.

This is a problem because evaporating water earlier suppose to be hotter, so this is not good news for those expecting the air to be colder!

Yet, if a person stand directly in front of the flowing air, he or she would feel the following:

  1. Colder, due to moisture is better at removing heat than air.
  2. Colder, if the person is not perspiring

Moisture (humidity) as it turns out, can remove heat much more efficiently than air. So it will feel colder to the skin. This is similar to using water jets showering fine droplets into the air (sometimes put behind a fan, used in many open air restaurants in hot countries). To measure this effect, the wet bulb of a thermometer can be used. It will reveal the coldest (or lowest) temperature the cooling effect can go. Assuming that their air is already full of moisture and no more evaporation can take place, then putting the thermometer directly at the flowing moist air would instantly reveal the wet bulb temperature. Nevertheless, whether the water is evaporating or not into the flowing air, the person standing in front of the blowing air would always feel cool. The reason is simple, flowing air, cools, even if the air is somewhat hot.

If however, that person were not being hit directly by the flowing air (standing outside the air path), it gets complicated. Since there is no more fast flowing air to mask the feeling of coldness, other factors would come into play. The cheap cooler, would now be exposed as what it actually is.
The person would feel the following:

  1. Hotter, due to the hot air and high humidity.
  2. Colder, due to moisture is better at removing heat than air.
  3. Colder, if the person is not perspiring.
  4. Hotter, if the person is perspiring.

The dominant effect, is clearly dependent on the many factors mentioned above, whether the person is perspiring or not, or whether the water is cold or not, and whether the water evaporates in great number or not.

Well, what if the experiment is conducted in a closed room? Would the air cooler cools the air in the room?

Picture 2

What will happen to the room, is it going to get colder as everyone assumed, or is it going to get hotter, or nothing?

When the water evaporates, the heat is transferred into the air, making the air hotter. The left behind water would get colder however. The hotter air and the colder water, would technically going to cancel each other out. This is because the hotter air will circulate through the room and would come back to the inlet of the unit, which now circulating colder water. So hotter air, is cooled by colder water, leaving no net effect, at first look. The humidity inside the room would reach 100% very quickly and things would start to change.

The room will start to get hotter and hotter. The reason?

The air-cooler requires electricity to operate, perhaps around 30 Watt. Where would this energy ends up? It will stay in the room for sure, and it is equivalent to heating the room using a heating coil of 30 Watt. The net effect? The room will get hotter and hotter.

Picture 3

In order to cool the room, energy must be expelled.  Using standard split refrigerant aircond would accomplish exactly this.

Picture 4

An air-conditioning unit is different however. It can transfer the heat outside of the room, thereby making the room colder. This is accomplished by absorbing the heat into the refrigerant. The liquid refrigerant is expanded just near the in-room cooler, changing into a gas due to a drop in pressure and becoming colder at the same time. Now, the cool gaseous refrigerant will absorb the heat in the room and will become hotter. It then moves out of the room to the external exchanger and then compressed by a compressor, heating the gas in the process. The gas which is at high pressure and temperature must now be cooled, so outside air is used to cool the hot gas via a heat exchanger, cooling the gas until it turns back into a liquid form. The cycle repeats as it sent to the room once more.

This process is rather energy intensive, so it is better to use other means of heat transfer ‘symbiosis’. For example, cold water used for washing can be utilized to cool the room too. One need to be creative on how to make full use of his or her resources, isn’t it?

Now to those who already have these cheap cooling units, what can they do?

The easiest and simplest solution is to put ice into the water tank. This will allow the air to exchange its heat, into the ice, which will turn into, well, water. As long as the water is much colder than the room’s air, cooling would take place.

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